A gas turbine engine generally includes, in serial flow communication, a gas generator compressor, a combustor, a gas generator turbine, and a power turbine. The combustor generates combustion gases that are channeled to the gas generator turbine where they are expanded to drive the gas generator turbine. Then, the combustion gases are channeled to the power turbine where they are further expanded to drive the power turbine. The gas generator turbine is coupled to the gas generator compressor via a gas generator shaft, and the power turbine is coupled to an output shaft via a power turbine shaft. The output shaft may be coupled to a load, such as main rotor blades of a helicopter.
Aircraft utilize an engine controller to determine an amount of fuel (e.g., fuel flow demand) the gas turbine engine needs in order to produce a desired power (thrust). In operation, the engine controller determines a tracking error between a reference speed of the gas turbine engine and an actual speed of the gas turbine engine. In modern FADEC systems, the tracking error represents a rate-based value, and the engine controller utilizes the tracking error to determine a rate of change of fuel flow demand. The engine controller then integrates the rate of change of fuel flow demand to determine an amount of fuel needed to produce the desired power. This approach provides integral control action and, as a result, provides superior error and bandwidth performance.
However, determining the fuel flow demand based on merely the tracking error is problematic, because loads enter the system as a disturbance on the powerplant (i.e., engine) during transient operation (e.g., acceleration, deceleration, etc.) of the aircraft. Disturbance rejection of this nature is very difficult to compensate without additional information. Thus, in an effort to improve operation of the aircraft during transient operation, the engine controller receives an operator command from an operator manipulated input device of the aircraft. The operator command represents a non-rate based value, and the engine controller utilizes the operator command to anticipate movement of the aircraft. However, as discussed below in more detail, FADEC systems cannot easily utilize the non-rate based operator command, because FADEC systems are rate-based systems.
More specifically, in operation, the engine controller utilizes both the tracking error and the operator command to determine the fuel flow demand of the aircraft, but the engine controller requires complex logic in order to merge the rate-based value (e.g., tracking error) with the non-rate based value (e.g., operator command). The complex logic, in addition to adding memory and computational load, is susceptible to forgetting the previous history and becoming confused so that governed speed is inconsistent. In addition, known methods for correcting these issues increase the complexity and require more memory, and therefore compound the problems.
Accordingly, a system and method for providing improved control of the fuel flow demand of a gas turbine engine of an aircraft would be welcomed in the technology. In particular, systems and methods that reduce the complexity of the logic utilized to determine the fuel flow demand would be beneficial.